Beam Forming Network (BFN) is a major sub-system of array antennas working in the microwave or millimetre-wave frequency range and numerous configurations are known.
In the following description we consider the BFN in transmit, meaning that it provides power to the array antenna. Obviously, the same BFN could be used in receive, based on the reciprocity principle.
For multiple beam applications, the BFN can be either orthogonal or coherent (in-phase).
Orthogonal BFNs such as Butler matrices (see J. Butler and R. Lowe, “Beam-Forming Matrix Simplifies Design of Electronically Scanned Antennas”, Electronic Design pp 170-173, April 1961) Nolen matrices (see J. C. Nolen, “Synthesis of Multiple Beam Networks for Arbitrary Illuminations”, PhD Thesis, Bendix Corporation, Radio Division, Baltimore, 21 Apr. 1965), etc. are interesting as they are lossless but this property imposes strong constraints on the amplitude and phase of the signals feeding the array.
For some applications, it may be convenient to have some control on the amplitude and/or phase. In this case, we prefer to use coherent (in-phase) BFNs associated to phase-shifters and eventually attenuators as described in document U.S. Pat. No. 3,868,695.
Document U.S. Pat. No. 3,868,695 describes a coherent BFN wherein each input is associated to one of the N beams produced by the array. The input power is divided into M signals of equal amplitude and phase corresponding to the number of radiating elements. A matrix of N×M amplitude and phase controls can be implemented to modify the characteristics of any signal. Then a combination network is used to direct the N signals towards the corresponding radiating element. The losses come mainly from this combination network as the combined signals have usually different frequencies. If tapered excitation laws are required to reduce side lobe levels, further losses must be added with attenuators at the amplitude and phase controls level. The combination and division networks are planar. All the combination networks (respectively division networks) are parallel to each other and orthogonal to all the division networks (respectively combination networks).
Recently, another coherent BFN has been introduced in the literature: the Coherently Radiating Periodic Structure (CORPS) BFN (see D. Betancourt and C. Del Rio, “A Beamforming Network for Multibeam Antenna Arrays Based on Coherent Radiating Periodic Structures”, EuCAP 2007, 11-16 Nov. 2007). It is inspired by the binomial excitation law in the case of mono-beam applications. In the case of multiple beam applications, the excitation law in amplitude is better described by a Gaussian tapering that highly depends on the number of layers. This natural amplitude tapering is interesting to reduce side lobe level and as a consequence interferer level in multiple beam applications with frequency reuse (SDMA).
FIG. 1 illustrates a CORPS BFN producing 3 beams and comprising 3 input ports (one port per beam), 5 output ports (3 per input port) and 2 layers, each layer comprising an alternate arrangement of power dividers (D) and power combiners (C). In this figure, the electrical paths followed by the signal entering at input port 2 are highlighted. With the configuration of FIG. 1 we can see how signals overlap at the output ports: some outputs only receive one of the input signals while some others combine 2 or even 3 input signals (see the curves represented at the top of FIG. 1 and illustrating the Gaussian amplitude distribution provided by the network).
The design of FIG. 1 may be of interest for an array in front of a parabolic reflector as the electromagnetic field amplitude localization in the focal plane directly impacts on the beam pointing direction, although in some cases we need similar overlaps between all the signals that are not possible with this planar configuration. In fact we see in FIG. 1 that the signals B1 and B2 in one hand and B2 and B3 one the other hand overlap in a similar way, but it is not the case of B1 and B3.
Furthermore, for conformal direct radiating arrays (DRA) applications like circular, cylindrical or conical array antennas, it may be also of interest to keep similar overlaps between adjacent input signals in multiple beam configurations that are also not accessible with this planar implementation.
In the specific case of circular arrays, this problem may be partially overcome with the architectures described in documents U.S. Pat. No. 3,573,837 and U.S. Pat. No. 4,316,192. First configuration enables one beam to be steered by simply switching between the different radiating elements of a circular array. The implementation of this solution for multiple beam applications becomes quickly complex. The second solution is quite similar except that the amplitude of the signals feeding the circular array is controlled by the combination of a Butler matrix and a row of phase shifters. This also leads to a quite complex structure that is usually preferred for a limited number of beams to be steered.